A bomb injured two Americans employed at the U.S. consulate in Peshawar, along with two local staff of the mission, when a suicide attacker rammed their vehicle Monday, officials said.

Two Pakistanis were killed at the site of the explosion, while 19 others were injured. The local police escorting the American vehicle were among the victims.

The ability of militants to hit the consulate staff at all is unusual. Extreme security measures guard the movement of U.S. diplomats and other American staff based at the heavily fortified consulate in Peshawar, a city in Pakistan's northwest which lies close to the lawless tribal area and the border with Afghanistan.

The powerful bomb left the American SUV a charred skeleton. But it was almost certainly an armored vehicle, which is what likely saved the lives of those riding inside. Police put the size of the bomb, which left a wide crater in the ground, at a hefty 100 kg. Local television channels showed a damaged American passport that belonged to one of the injured staff.

The consulate employees had “non life-threatening injuries,” said Rian Harris, a spokesperson for the U.S. embassy in Islamabad.

A State Department statement described the attack as a “heinous act”, and said that U.S. stood ready to work with Pakistani authorities “so that the perpetrators can be brought to justice.”

Mian Iftikhar Hussain, the information minister of the provincial government in the northwest, said that the car of the bomber managed to slip in between the American vehicle and its escort.

“The message of these attackers is that it is not safe for any foreign country to have a consulate here,” said Hussain.

Peshawar is a highly volatile city, which is often the site for bombings aimed against Pakistani security personnel. The consulate, apart from diplomatic activity in Peshawar, is presumed to host a major spying station for the United States. Senior al Qaida operatives as well as militants of the Haqqani network, an Afghan insurgent group, are based in the tribal area, which serves as a buffer zone between Pakistan and Afghanistan. Missile strikes by U.S. drone aircraft often target suspected extremists in the tribal area.

In 2010, the consulate came under a multi-pronged gun and bomb assault, while in 2008, the vehicle of the top U.S. diplomat based there was attacked by gunmen just after she left her house, but she was unhurt.